This invention relates to a semiconductor device with high gain and high withstand voltage, more specifically to a bipolar semiconductor device exhibiting high gain and high withstand voltage in a high frequency range.
In order to obtain high gain in the high frequency range, a bipolar transistor must be so designed as to have optimum characteristic elements (parameters). Above all, it is to be desired that D.C. current-amplification factor h.sub.FE should be high enough. In manufacturing a single transistor with a high current-amplification factor, however, collector-emitter withstand voltage V.sub.CEO would be lowered to cause difficulties in practical use. The withstand voltage V.sub.CEO has the following connection with collector-base withstand voltage V.sub.CBO and current-amplification factor h.sub.FE n. That is
V.sub.CEO .alpha.V.sub.CBO/h.sbsb.FE.sub.n.
Here n is a constant determined according to the semiconductor manufactured. As may be understood from the above expression, V.sub.CEO decreases as h.sub.FE increases. Determined mainly by the specific resistance of a collector layer, V.sub.CBO is set at an optimum value by means of the supply voltage used, and is prohibited from being unduly increased. V.sub.CEO may, therefore, be increased by reducing h.sub.FE, though, in such case, the gain will be lowered to constitute an obstacle to use. It is generally known that, in a low frequency range, h.sub.FE of the transistor device is increased synthetically by Darlington-connecting a plurality of transistors with relatively low h.sub.FE. In such Darlington connection, apparent h.sub.FE is equal to the product of the respective current-amplification factors of a plurality of series-connected transistors. A high-frequency, high-output transistor needs to use a shallow collector-base junction, requiring a very wide safe operation range. Accordingly, current concentration may result in the Darlington connection, and, in a high-frequency transistor, the inductance of a connecting lead will affect the transistor device as parasitic impedance. Thus, this type of Darlington connection would not be able to provide satisfactory high frequency response.